Magnetic resonance imaging system and method for rapid shutdown and recharge of a superconducting magnet

ABSTRACT

A magnetic resonance imaging (MRI) system includes a set of magnet coils for generating a magnetic field. The set of magnet coils are composed of a superconducting material. The system further includes a mechanical cryocooler in thermal contact with the set of magnet coils and operable to reduce and maintain a temperature of the set of magnet coils below a transition temperature of the superconducting material, and an energy storage device coupled to the set of magnet coils. The energy storage device may be capable of receiving and storing energy dissipated from the set of magnet coils during rapid shutdown of the set of magnet coils. The system may also include a controller coupled to the energy storage device. The controller may be programmed to recharge the set of magnet coils using the energy stored in the energy storage device during the rapid shutdown of the set of magnet coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/162,051 filed Jan. 29, 2021. The contents of the application areherein incorporated by reference in its entirety.

BACKGROUND

The field of the invention is systems and methods for magnetic resonanceimaging (“MRI”). More particularly, the invention relates to systems andmethods for MRI in which the magnetic field of the MRI scanner can berapidly shutdown and recharged as needed.

MRI systems typically utilize one of two types of magnet assemblies togenerate the strong, main magnetic field used for imaging. One typegenerates the main magnetic field using permanent magnets. This type ofsystem is less popular because the magnetic field strengths that can beachieved with such systems is limited. Moreover, these systems tend tobe extremely heavy and are very sensitive to temperature fluctuations.Permanent magnets also cannot be turned off, so there is no way toremove the magnetic field.

The second type of MRI system generates the main magnetic field using asuperconducting electromagnet. Using superconducting magnets allows highcurrent densities through the conductors of the electromagnet withoutpower dissipation, which in turn enables the ability to achieve highmagnetic field strengths. For the magnet to be superconducting, themagnet coils must be cooled to extremely low temperatures (e.g., about 4K).

One method used to cool the superconducting magnet coils to this lowtemperature is done by immersing the conductor in a liquid helium bath.These superconducting systems tend to be very expensive because of thehigh cost of the liquid cryogens (e.g., liquid helium). Furthermore, itis not easy to rapidly turn on or off the magnetic fields generated bythese systems. For example, to rapidly turn off the magnetic field(referred to herein as a “quench”) typically requires heating up theconductive magnet coils so that they develop resistance that candissipate their stored energy. This resistance produces heat that causesthe liquid cryogen, which is providing the cooling, to convert torapidly expanding gas. This boiling-off of the liquid cryogen removesthe cooling capability of the system, and thus the magnetic fieldgenerated by the magnet coils. But, current cannot be restored in themagnetic coils and the magnet field cannot be regenerated until theliquid cryogen is replaced and the magnet coils are cooled back down tosuperconducting temperatures, a process that normally involves multipledays and significant expense. Furthermore, there is a risk of thesuperconducting magnet coils to be damaged during the rapid heat up ordisplaced from their ideal position. The consequences of damage to themagnet coils can be as extreme as needing to be completely replacedafter a quench.

Alternatively, current can be removed or added to superconducting magnetsystems very slowly without causing enough heating to boil off theliquid cryogen. In these situations, it takes many hours to completelyadd or remove the current, making rapid turning the magnetic field on oroff (e.g., an emergency shutdown) in this manner not feasible.

For safety reasons, it would be beneficial for an MRI scanner to becapable of having the magnetic field rapidly turned off. For example,large metallic objects being attracted by the strong magnetic field isone of the primary risks associated with these devices. In casesrequiring emergency personnel, that may, for example, require oxygentanks, or, in situations where someone is physically “pinned” to themagnet by a large metallic object, the magnet must be turned off in avery fast manner. Traditional superconducting magnets have implemented amechanism to rapidly turn off the magnetic field in an emergencysituation by “quenching” the magnet in the manner described above, whereall liquid cryogens are boiled off very rapidly. Quenching the magnet,however, requires a time consuming and expensive replacement of theliquid cryogens before the magnetic field can be reestablished.

SUMMARY OF THE DISCLOSURE

In accordance with an embodiment, a magnetic resonance imaging (MRI)system includes a set of magnet coils for generating a magnetic field.The set of magnet coils are composed of a superconducting material. Thesystem further includes a mechanical cryocooler in thermal contact withthe set of magnet coils and operable to reduce and maintain atemperature of the set of magnet coils below a transition temperature ofthe superconducting material and an energy storage device coupled to theset of magnet coils and configured to receive and store energydissipated from the set of magnet coils during a rapid shutdown of theset of magnet coils.

In accordance with another embodiment, a method for rapid shutdown andrecharging of a superconducting magnet includes dissipating energy froma set of magnet coils in the superconducting magnet into an energystorage device coupled to the set of magnet coils based on a rapidshutdown condition, storing the dissipated energy in the energy storagedevice, determining a status of the rapid shutdown condition, andrecharging the set of magnet coils using the energy stored in the energystorage device based on the status of the rapid shutdown condition.

In accordance with another embodiment, a system for rapid shutdown andrecharging of a superconducting magnet includes an energy storage devicecoupled to the superconducting magnet and configured to receive andstore energy dissipated from the superconducting magnet based on a rapidshutdown condition, and a controller coupled to the energy storagedevice and programmed to recharge the superconducting magnet using theenergy stored in the energy storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will hereafter be described with reference to theaccompanying drawings, wherein like reference numerals denote likeelements.

FIG. 1 is a block diagram of a magnetic resonance imaging (“MRI”) systemcapable of rapid shutdown and recharge of a superconducting magnet inaccordance with an embodiment;

FIG. 2 illustrates a method for rapid shutdown and recharge of asuperconducting magnet in accordance with an embodiment;

FIG. 3 is a block diagram of an MRI system capable of rapid shutdown ofa superconducting magnet in accordance with an embodiment; and

FIG. 4 is a block diagram of an MRI system capable of rapid shutdown andrecharge of a superconducting magnet in accordance with an embodiment.

DETAILED DESCRIPTION

The present disclosure describes systems and methods for rapid magneticfield shutdown and recharging in a magnetic resonance imaging (“MRI”)system that includes a superconducting magnet cooled by a mechanicalcryocooler.

Recently, there have been advances in superconductors andsuperconducting magnet design aimed at reducing the amount of expensiveliquid cryogen required to achieve and maintain superconductingproperties. These advances include the development of high temperaturesuperconductors that are conductors that become superconducting attemperatures higher than 4 K. Currently, reasonable high temperaturesuperconductors can operate at 10 K; although, some materials candemonstrate superconducting properties at temperatures as high as 30 K.Furthermore, there have been recent proposals on cryogen-free magnetdesigns that use a cryocooler to cool the magnet coil conductors throughthermal contact rather than immersing the magnet coils within a liquidhelium bath.

The systems and methods described here are based on such a cryogen freesuperconducting magnet design using traditional, or high temperature,superconductors where the main magnetic field can be turned off in ashort amount of time. For instance, the magnetic field can be turned offin an amount of time comparable to a typical amount of time atraditional “quench” would take, e.g., less than 10 seconds.

The MRI system described here uses a mechanical cryocooler (or coldhead) that is in thermal contact with the conductors in asuperconducting magnet to cool them to temperatures approaching 4 K.Here, thermal contact can include direct or indirect contact, throughwhich thermal energy can be transferred or conducted. Thesuperconducting material used for the magnet design preferably maintainssuperconducting properties up to temperatures approaching 8 K. In thedescribed system, current density can be removed from the conductivewindings of the magnet coils in a rapid manner by introducing one or acombination of a power supply source, a resistive load and an externalenergy source. In one embodiment, a power supply source introduced intothe circuit (e.g., by means of a superconducting switch) may be used tosupply current to the magnet coils. Supplying current to the magnetcoils introduces heat into the system, which can be removed using thethermal cooling capacity of the mechanical cryocooler (or cold head). Inanother embodiment, a resistive load with a large thermal mass may beintroduced into the circuit (e.g., by means of a superconducting switch)and the majority of the energy stored in the superconducting magnet maybe dissipated to this load rather than the magnet coils of thesuperconducting magnet during a rapid shutdown (or ramp down) to turnoff the magnetic field. In yet another embodiment, an external energystorage device may be introduced into the circuit (e.g., by means of asuperconducting switch) and may be used to store all, or part of theenergy contained within the superconducting magnet coils that isdissipated during a rapid shutdown (or ramp down) to turn off themagnetic field. As mentioned, in other embodiments, combinations of thepower supply source, resistive load and external energy storage devicemay be used for rapid shutdown. In addition, one or a combination of thepower supply source, the resistive load and the external energy storagedevice may be used to recharge the magnet coils after a rapid shutdown.

In this system, the rate of energy exchange change (and thus the rate ofmagnetic field change) can be controlled so that the temperature of theconductor does not exceed a predetermined threshold that couldpotentially cause irreversible damage. For example, the predeterminedthreshold may be the superconducting transition point of the magnet coilmaterial. In this manner, there is no rapid resistance changes in theconductor to cause an uncontrolled loss of magnetic field (i.e., aquench). In another example, the predetermined threshold may be a largertemperature than the superconducting transition point, for example, 20K, so long as the temperature a) doesn't cause significant damage to thewire or magnet structure; and b) doesn't require a significant amount oftime to cool back down to superconducting temperature (˜4-5 K).

Referring now to FIG. 1 , a magnetic resonance imaging system 10generally includes a magnet assembly 12 for providing a magnetic field14 that is substantially uniform within a bore 16 that may hold asubject 18 or other object to be imaged. The magnet assembly 12 supportsa radio frequency (“RF”) coil (not shown) that may provide an RFexcitation to nuclear spins in the object or subject (not shown)positioned within the bore 16. The RF coil communicates with an RFsystem 20 producing the necessary electrical waveforms, as is understoodin the art. The magnet assembly 12 also supports three axes of gradientcoils (not shown) of a type known in the art, and which communicate witha corresponding gradient system 22 providing electrical power to thegradient coils to produce magnetic field gradients, G_(x), G_(y), andG_(z) over time.

A data acquisition system 24 connects to RF reception coils (not shown)that are supported within the magnet assembly 12 or positioned withinbore 16. The RF system 20, gradient system 22, and data acquisitionsystem 24 each communicates with a controller 26 that generates pulsesequences that include RF pulses from the RF system 20 and gradientpulses from gradient system 22. The data acquisition system 24 receivesmagnetic resonance signals from the RF system 20 and provides themagnetic resonance signals to a data processing system 28, whichoperates to process the magnetic resonance signals and to reconstructimages therefrom. The reconstructed images can be provided to a display30 for display to a user.

The magnet assembly 12 includes one or more magnet coils 32 housed in avacuum housing 34, which generally provides a cryostat for the magnetcoils 32, and mechanically cooled by a mechanical cryocooler 36, such asa Gifford-McMahon (“GM”) cryocooler or a pulse tube cryocooler. In oneexample configuration, the cryocooler can be a Model RDK-305Gifford-McMahon cryocooler manufactured by Sumitomo Heavy Industries(Japan). In general, the cryocooler 36 is in thermal contact with themagnet coils 32 and is operable to lower the temperature of the magnetcoils 32 and to maintain the magnet coils 32 and a desired operatingtemperature. In some embodiments the cryocooler 36 includes a firststage in thermal contact with the vacuum housing 34 and a second stagein thermal contact with the magnet coils 32. In these embodiments, thefirst stage of the cryocooler 36 maintains the vacuum housing 34 at afirst temperature and the second stage of the cryocooler 36 maintainsthe magnet coils 32 at a second temperature that is lower than the firsttemperature.

The magnet coils 32 are composed of a superconducting material andtherefore provide a superconducting magnet. The superconducting materialis preferably selected to be a material with a suitable criticaltemperature such that the magnet coils 32 are capable of achievingdesired magnetic field strengths over a range of suitable temperatures.As one example, the superconducting material can be niobium (“Nb”),which has a transition temperature of about 9.2 K. As another example,the superconducting material can be niobium-titanium (“NbTi”), which hasa transition temperature of about 10 K. As still another example, thesuperconducting material can be triniobium-tin (“Nb₃Sn”), which has atransition temperature of about 18.3 K.

The choice of superconducting material will define the range of magneticfield strengths achievable with the magnet assembly 12. Preferably, thesuperconducting material is chosen such that magnetic field strengths upto about 3.0 T can be achieved over a range of temperatures that can besuitably achieved by the cryocooler 36. In some configurations, however,the superconducting material can be chosen to provide magnetic fieldstrengths higher than 3.0 T.

The cryocooler 36 is operable to maintain the magnet coils 32 at anoperational temperature at which the magnet coils 32 aresuperconducting, such as a temperature that is below the transition, orcritical, temperature for the material of which the magnet coils 32 arecomposed. As one example, a lower operational temperature limit can beabout 4 K and an upper operational temperature limit can be at or nearthe transition, or critical, temperature of the superconducting materialof which the magnet coils 32 are composed.

The current density in the magnet coils 32 in the MRI system 10 may becontrollable to rapidly ramp up or ramp down the magnetic field 14generated by the magnet assembly 12 while controlling the temperature ofthe magnet coils 32 with the cryocooler 36 to keep the temperature belowthe transition temperature of the superconducting material of which themagnet coils 32 are composed. As one example, the magnetic field 14 canbe ramped up or ramped down on the order of minutes, such as fifteenminutes or less.

In general, the current density in the magnet coils 32 can be increasedor decreased by connecting the magnet coils 32 to a circuit with a powersupply 38 that is in electrical communication with the magnet coils 32via a switch 40 and operating the power supply 38 to increase ordecrease the current in the connected circuit. The switch 40 isgenerally a superconducting switch that is operable between a first,closed, state and a second, open, state.

When the switch 40 is in its open state, the magnet coils 32 are in aclosed circuit, which is sometimes referred to as a “persistent mode.”In this configuration, the magnet coils 32 are in a superconductingstate so long as the temperature of the magnet coils 32 is maintained ata temperature at or below the transition temperature of thesuperconducting material of which they are composed.

When the switch 40 is in the closed state, however, the magnet coils 32and the power supply 38 can be placed in a connected circuit, and thecurrent supplied by the power supply 38 and the current in the magnetcoils 32 will try to equalize. For instance, if the power supply 38 isoperated to supply more current to the connected circuit, the current inthe magnet coils 32 will increase, which will increase the strength ofthe magnetic field 14. On the other hand, if the power supply 38 isoperated to decrease the current in the connected circuit, the currentin the magnet coils 32 will decrease, which will decrease the strengthof the magnetic field 14.

It will be appreciated by those skilled in the art that any suitablesuperconducting switch can be used for selectively connecting the magnetcoils 32 and power supply 38 into a connected circuit; however, as onenon-limiting example, the switch 40 may include a length ofsuperconducting wire that is connected in parallel to the magnet coils32 and the power supply 38. To operate such a switch 40 into its closedstate, a heater in thermal contact with the switch 40 is operated toraise the temperature of the superconducting wire above its transitiontemperature, which in turn makes the wire highly resistive compared tothe inductive impedance of the magnet coils 32. As a result, very littlecurrent will flow through the switch 40. The power supply 38 can then beplaced into a connected circuit with the magnet coils 32. When in thisconnected circuit, the current in the power supply 38 and the magnetcoils 32 will try to equalize; thus, by adjusting the current suppliedby the power supply 38, the current density in the magnet coils 32 canbe increased or decreased to respectively ramp up or ramp down themagnetic field 14. To operate the switch 40 into its open state, thesuperconducting wire in the switch 40 is cooled below its transitiontemperature, which places the magnet coils 32 back into a closedcircuit, thereby disconnecting the power supply 38 and allowing all ofthe current to flow through the magnet coils 32.

When the magnet coils 32 are in the connected circuit with the powersupply 38, the temperature of the magnet coils 32 will increase as thecurrent in the connected circuit equalizes. Thus, the temperature of themagnet coils 32 should be monitored to ensure that the temperature ofthe magnet coils 32 remains below the transition temperature for thesuperconducting material of which they are composed. Because placing themagnet coils 32 into a connected circuit with the power supply 38 willtend to increase the temperature of the magnet coils 32, the rate atwhich the magnetic field 14 can be ramped up or ramped down will dependin part on the cooling capacity of the cryocooler 36. For instance, acryocooler with a larger cooling capacity will be able to more rapidlyremove heat from the magnet coils 32 while they are in a connectedcircuit with the power supply 38.

The power supply 38 and the switch 40 operate under control from thecontroller 26 to provide current to the magnet coils 32 when the powersupply 38 is in a connected circuit with the magnet coils 32. A currentmonitor 42 measures the current flowing to the magnet coils 32 from thepower supply 38, and a measure of the current can be provided to thecontroller 26 to control the ramping up or ramping down of the magneticfield 14. In some configurations, the current monitor 42 is integratedinto the power supply 38.

A temperature monitor 44 is in thermal contact with the magnet assembly12 and operates to measure a temperature of the magnet coils 32 inreal-time. As one example, the temperature monitor 44 can include athermocouple temperature sensor, a diode temperature sensor (e.g., asilicon diode or a GaAlAs diode), a resistance temperature detector(“RTD”), a capacitive temperature sensor, and so on. RTD-basedtemperature sensors can be composed of ceramic oxynitride, germanium, orruthenium oxide. The temperature of the magnet coils 32 is monitored andcan be provided to the controller 26 to control the ramping up orramping down of the magnetic field 14.

In operation, the controller 26 is programmed to ramp up or ramp downthe magnetic field 14 of the magnet assembly 12 in response toinstructions from a user. As mentioned above, the magnetic field 14 canbe ramped down by decreasing the current density in the magnet coils 32by supplying current to the magnet coils 32 from the power supply 38 viathe switch 40, which is controlled by the controller 26. Likewise, thestrength of the magnetic field 14 can be ramped up by increasing thecurrent density in the magnet coils 32 by supplying current to themagnet coils 32 from the power supply 38 via the switch 40, which iscontrolled by the controller 26.

The controller 26 is also programmed to monitor various operationalparameter values associated with the MRI system 10 before, during, andafter ramping the magnetic field 14 up or down. As one example, asmentioned above, the controller 26 can monitor the current supplied tothe magnet coils 32 by the power supply 38 via data received from thecurrent monitor 42. As another example, as mentioned above, thecontroller 26 can monitor the temperature of the magnet coils 32 viadata received from the temperature monitor 44. As still another example,the controller 26 can monitor the strength of the magnetic field 14,such as by receiving data from a magnetic field sensor, such as a Hallprobe or the like, positioned in or proximate to the bore 16 of themagnet assembly 12.

As mentioned above, certain conditions or situations may require thatthe magnetic field 14 of the magnet assembly 12 be shut down (or turnedoff) rapidly. For example, an emergency situation may be created by alarge metallic object being attracted by the strong magnetic field ofthe magnet assembly 12. In one embodiment, the power supply source 38may also be used to rapidly shutdown the magnetic field 14 of the magnetassembly 12 in response to a shutdown condition. As discussed above, thepower supply source 38 may be connected to the magnet coils 32 andoperated to remove or decrease the current in the magnet coils 32. Thecryocooler 36 may be used to remove heat generated by the magnet coils32 as the current in the magnet coils 32 decreases. In an embodiment,the temperature monitor 44 may be used to measure a temperature of themagnet coils 32 in real-time. The controller 26 may be configured torapidly shutdown (or turn off) the magnet field 14 of the magnetassembly 12 in response to instructions from a user. The user mayprovide instructions to the controller based on the presence of ashutdown condition.

In another embodiment, a rapid shutdown (e.g., an emergency shutdown) ofthe magnet field of the magnet assembly 12 may be performed using anenergy storage device 46 that is coupled to the magnet coils 32 and thecontroller 26. In one embodiment, the energy storage device may be aninductive load. For example, the inductive load may be a secondsuperconducting system. The second superconducting system may bethermally coupled to the cryocooler 36 of MRI system 10 and cooled bythe cryocooler 36. In another embodiment, the energy storage device 46may be a battery. The energy storage device 46 may be coupled to themagnet coils 32 using a superconducting switch 50. The superconductingswitch 50 may be controlled using, for example, controller 26 toselectively connect the energy storage device 46 and the magnet coils 32into a connected circuit. In an embodiment, the superconducting switch50 may be any suitable superconducting switch that can be used forselectively connecting the magnet coils 32 and energy storage device 46into a connected circuit. For example, the superconducting switch 50 maybe switched between an open state and a closed state as described in thenon-limiting example mentioned above.

The energy storage device 46 may be used to store all, or a part of, theenergy contained in the magnet coils 32 so that the current density isremoved from the magnet coils 32 and the magnetic field 14 turned off.In other words, the energy from the magnet coils 32 may be dissipatedinto the energy storage device 46 during the rapid shutdown of themagnetic field 46. In an embodiment, the magnetic field 14 may be turnedoff in a short amount of time, for example, in an amount of timecomparable to a typicality amount of time a transitional “quench” wouldtake (e.g., less than 10 seconds). The controller 26 may be configuredto rapidly shutdown (or turn off) the magnet field 14 of the magnetassembly 12 in response to instructions from a user. The user mayprovide instructions to the controller 26 based on the presence of ashutdown condition.

As mentioned above, the rate of energy exchange change (and thus therate of magnetic field change) can be controlled so that the temperatureof the conductor (magnet coils 32) does not exceed a predeterminedthreshold that could potentially cause irreversible damage. For example,the predetermined threshold may be the superconducting transition pointof the magnet coil 32 material. In another example, the predeterminedthreshold may be a larger temperature than the superconductingtransition point, for example, 20 K. In an embodiment, the temperaturemonitor 44 may be used to measure a temperature of the magnet coils 32in real-time. The temperature of the magnet coils 32 may be monitoredand the temperature may be provided to the controller 26 to control theraid shutdown of the magnetic field 14.

After the magnetic field 14 has been shut down (or turned off), thecondition(s) that led to the need for the rapid shutdown may beresolved. Once the rapid shutdown condition has been resolved, theenergy stored in the energy storage device 46 from the shutdown of themagnetic field 14 may be used to fully, or partially, recharge themagnet coils 32. The controller 26 may be configured to recharge themagnet coils 32 using the energy stored in the energy storage device 46from the shutdown of the magnetic field 14 in response to instructionsfrom a user. For example, the energy storage device 46 and thesuperconducting switch 50 may operate under control from the controller26 to provide the energy stored in the energy storage device 46 to themagnet coils 32 when the energy storage device 46 is in a connectedcircuit with the magnet coils 32.

FIG. 2 illustrates a method for rapid shutdown and recharge of asuperconducting magnet in accordance with an embodiment. At block 202,energy from a set of magnet coils in a magnet assembly of an MRI systemis dissipated to an energy storage device coupled to the magnet coils.The energy is dissipated based on a rapid shutdown condition, forexample, the present of a large metallic object that is attracted by thestrong magnetic field of the MRI system. In an embodiment, a user mayprovide instructions to the MRI system to rapidly shutdown of themagnetic field of the magnet assembly. In one example, a superconductingswitch may be used to connect the energy storage device to the magnetcoils. During the shutdown of the magnetic field, current density isremoved from the magnet coils and the energy dissipated to the energystorage device. In an embodiment, the magnetic field may be turned offin a short amount of time, for example, in an amount of time comparableto a typicality amount of time a transitional “quench” would take (e.g.,less than 10 seconds). As mentioned above, the rate of energy exchangechange (and thus the rate of magnetic field change) can be controlled sothat the temperature of the conductor does not exceed a predeterminedthreshold that could potentially cause irreversible damage. In anembodiment, a temperature monitor may be used to measure a temperatureof the magnet coils in real-time. The temperature of the magnet coilsmay be monitored and the temperature may be provided to a controller ofthe MRI system to control the raid shutdown of the magnetic field 14

At block 204, the energy dissipated from the magnet coils is stored inthe energy storage device. The energy storage device may be, forexample, an inductive load or a battery. After the magnetic field hasbeen turned off, the status of the rapid shutdown condition isdetermined at block 206. If the rapid shutdown condition has not beenresolved at block 208, the magnetic field will remain turned off untilthe issue is resolved. If the rapid shutdown condition has been resolvedat block 208, the magnet coils of the magnet assembly may be rechargedusing the energy stored in the energy storage device at block 210. In anembodiment, a user may provide instructions to the MRI system torecharge of the magnet coils of the magnet assembly.

In another embodiment, a rapid shutdown (e.g., an emergency shutdown) ofthe magnet coils 32 may be performed using a resistive load coupled tothe magnet coils 32. FIG. 3 is a block diagram of an MRI system capableof rapid shutdown of a superconducting magnet. The elements andoperation of MRI system 10 shown in FIG. 3 are similar to the MRI systemdescribed above with respect to FIG. 1 . In FIG. 3 , the MRI system 10includes a resistive load 48 is coupled to magnet coils 32 of a magnetassembly 12. In an embodiment, the resistive load 48 has a large thermalmass. The resistive load 48 may be coupled to the magnet coils 32 usinga superconducting switch 52. The superconducting switch 52 may becontrolled using, for example, controller 26 to selectively connect theresistive load 48 and the magnet coils 32 into a connected circuit. Inan embodiment, the superconducting switch 52 may be any suitablesuperconducting switch that can be used for selectively connecting themagnet coils 32 and resistive load 48 into a connected circuit. Forexample, the superconducting switch 52 may be switched between an openstate and a closed state as described in the non-limiting examplementioned above. Energy from the magnet coils 32 may be dissipated tothe resistive load 48 during rapid shutdown of the magnetic field 14. Inan embodiment, the magnetic field 14 may be turned off in a short amountof time, for example, in an amount of time comparable to a typicalityamount of time a transitional “quench” would take (e.g., less than 10seconds). As mentioned above, the rate of energy exchange change (andthus the rate of magnetic field change) can be controlled so that thetemperature of the conductor (magnet coils 32) does not exceed apredetermined threshold that could potentially cause irreversibledamage. In an embodiment, a temperature monitor 44 may be used tomeasure a temperature of the magnet coils 32 in real-time. Thetemperature of the magnet coils 32 may be monitored and the temperaturemay be provided to a controller 26 to control the rapid shutdown of themagnetic field 14. The controller 26 may be configured to rapidlyshutdown (or turn off) the magnet field 14 of the magnet assembly 12 inresponse to instructions from a user. The user may provide instructionsto the controller based on the presence of a shutdown condition.

In yet another embodiment, a resistive load may be used in combinationwith an energy storage device to rapidly shutdown and recharge themagnet coils as shown in FIG. 4 . The elements and operation of MRIsystem 10 shown in FIG. 4 are similar to the MRI system described abovewith respect to FIG. 1 . In FIG. 4 , the MRI system includes a resistiveload 48 is coupled to magnet coils 32 of a magnet assembly 12 and theresistive load 48 is also coupled to an energy storage device 46. Theenergy storage device 46 is coupled to a controller 26. In oneembodiment, the energy storage device may be an inductive load. Forexample, the inductive load may be a second superconducting system. Thesecond superconducting system may be thermally coupled to the cryocooler36 of MRI system 10 and cooled by the cryocooler 36. In anotherembodiment, the energy storage device 46 may be a battery. The energystorage device 46 may be coupled to the magnet coils 32 using asuperconducting switch 50 and the resistive load 48 may be coupled tothe magnet coils 32 using a superconducting switch 52. Thesuperconducting switches 50, 52 may be controlled using, for example,controller 26 to selectively connect the energy storage device 46 andthe resistive load 48, respectively, and the magnet coils 32 into aconnected circuit. In an embodiment, the superconducting switches 50, 52may be any suitable superconducting switch that can be used forselectively connecting the magnet coils 32 and resistive load 48 into aconnected circuit. For example, the superconducting switches 50, 52 maybe switched between an open state and a closed state as described in thenon-limiting example mentioned above.

Energy from the magnet coils 32 may be dissipated to the resistive load48 during rapid shutdown of the magnetic field 14. The controller 26 maybe configured to rapidly shutdown (or turn off) the magnet field 14 ofthe magnet assembly 12 in response to instructions from a user. The usermay provide instructions to the controller 26 based on the presence of ashutdown condition. Thermal energy (or heat) dissipated by the resistiveload 48 may be used to charge the energy storage device 46. As mentionedabove, the rate of energy exchange change (and thus the rate of magneticfield change) can be controlled so that the temperature of the conductor(magnet coils 32) does not exceed a predetermined threshold that couldpotentially cause irreversible damage. In an embodiment, a temperaturemonitor 44 may be used to measure a temperature of the magnet coils 32in real-time. The temperature of the magnet coils 32 may be monitoredand the temperature may be provided to a controller 26 to control theraid shutdown of the magnetic field 14.

After the magnetic field 14 has been shut down (or turned off), thecondition(s) that led to the need for the rapid shutdown may beresolved. Once the rapid shutdown condition has been resolved, theenergy stored in the energy storage device 46 from the resistive load 48may be used to fully, or partially, recharge the magnet coils 32. Thecontroller 26 may be configured to recharge the magnet coils 32 usingthe energy stored in the energy storage device 46 in response toinstructions from a user. For example, the energy storage device 46 andthe superconducting switch 50 may operate under control from thecontroller 26 to provide the energy stored in the energy storage device46 to the magnet coils 32 when the energy storage device 46 is in aconnected circuit with the magnet coils 32.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

We claim:
 1. A system for rapid shutdown and recharging of asuperconducting magnet, the system comprising: an energy storage devicecoupled to the superconducting magnet and configured to receive andstore energy dissipated from the superconducting magnet based on a rapidshutdown condition; a controller coupled to the energy storage deviceand programmed to recharge the superconducting magnet using the energystored in the energy storage device in response to the rapid shutdowncondition; and a mechanical cryocooler thermally coupled to thesuperconducting magnet superconducting magnet and configured to reduceand maintain a temperature of the superconducting magnet; wherein theenergy storage device is an inductive load and the inductive load is asuperconducting system, and wherein the superconducting system isthermally coupled to the mechanical cryocooler which is furtherconfigured to reduce and maintain a temperature of the superconductingsystem.
 2. The system according to claim 1, further comprising aresistive load coupled to the superconducting magnet and the energystorage device, the resistive load configured to receive energydissipated from the superconducting magnet based on the rapid shutdowncondition and the energy storage device is further configured to becharged using thermal energy dissipated from the resistive load.